Impedance measuring circuit and capacitance measuring circuit

ABSTRACT

An electrostatic capacitance detection circuit  10  comprises an AC voltage generator  11,  an operational amplifier  14  of which non-inverting input terminal is connected to specific potential (a ground in this example), an impedance converter  16,  a resistance (R 1 )  12  connected between the AC voltage generator  11  and an inverting input terminal of the operational amplifier  14,  a resistance (R 2 )  13  connected between the inverting input terminal of the operational amplifier  14  and an output terminal of the impedance converter  16,  and an impedance element (a capacitor)  15  connected between an output terminal of the operational amplifier  14  and an input terminal of the impedance converter  16.  A capacitor to be detected  17  is connected between the input terminal of the impedance converter  16  and the specific potential.

TECHNICAL FIELD

The present invention relates to a circuit that detects impedance,especially relates to the circuit that detects very small impedance withhigh accuracy.

BACKGROUND ART

As a prior art of an electrostatic capacitance detection circuit, thatdescribed in Japanese Laid-Open Patent Application H09-280806 gazettecan be cited. FIG. 1 is a circuit diagram that shows this electrostaticcapacitance detection circuit. In this detection circuit, a capacitivesensor 92 comprised of electrodes 90 and 91 is connected to an invertinginput terminal of an operational amplifier 95 via a signal line 93. Anda capacitor 96 is connected between an output terminal of thisoperational amplifier 95 and the said inverting input terminal, andfurther an AC voltage Vac is applied to a non-inverting input terminal.Also, the said signal line 93 is wrapped up by a shield line 94 andshielded electrically against disturbance noise. And this shield line 94is connected to the non-inverting input terminal of the operationalamplifier 95. Output voltage Vd is obtained from an output terminal ofthe said operational amplifier 95 via a transformer 97.

In this detection circuit, the inverting input terminal and thenon-inverting input terminal of the operational amplifier 95 are in animaginary short status, so that the signal line 93 connected to theinverting input terminal and the shied line 94 connected to thenon-inverting input terminal have the almost same potential. Thereby,the signal line 93 is guarded by the shield line 94, that is, straycapacitance between the signal line 93 and the shield line 94 iscanceled, and the output voltage Vd, which is unlikely to be affected bythe stray capacitance, can be obtained.

According to this kind of conventional art, when capacitance of thecapacitive sensor 92 is big to some extend, it is indeed possible toobtain accurate output voltage Vd, which is not affected by the straycapacitance between the signal line 93 and the shield line 94. However,when very small capacitance, which equals to or is less than an order ofseveral pF or fF (femtofarad), is detected, an error is increased.

Also, depending on a frequency of the AC voltage Vac applied, a subtledisplacement of a phase and amplitude consequently arises between thevoltage of the inverting input terminal and that of the non-invertinginput terminal, which are in the imaginary short status, due to atracking error in the operational amplifier 59, and thereby thedetection error becomes bigger.

On the other hand, for lightweight and small audio communication devicesrepresented by a mobile phone or the like, there has been a demand of acompact amplifier circuit that sensitively and faithfully transformssounds detected by a capacitive sensor such as a capacitor microphoneinto an electric signal. If it is possible to accurately detect verysmall capacitance that equals to or is less than several pF or fF and/orits change, a high performance microphone that can detect sounds with avery high level of sensitivity and fidelity is realized, and therebyperformance for picking up sounds by the audio communication devicessuch as a mobile phone will make rapid progress.

This invention is devised in view of the above-mentioned situation, andaims at providing a detection circuit of impedance includingelectrostatic, which is capable of accurately detecting very smallcapacitance, and suitable to detect capacitance of a capacitive sensorsuch as a capacitor microphone used for lightweight and compact audiocommunication devices.

DISCLOSURE OF INVENTION

In order to achieve above objectives, the electrostatic capacitancedetection circuit according to the present invention is an impedancedetection circuit that outputs a detection signal corresponding toimpedance of an impedance element to be detected, comprising: animpedance converter of which input impedance is high and outputimpedance is low; a first capacitive impedance element; a firstoperational amplifier; a voltage generator that applies at least ACvoltage or DC voltage to the first operational amplifier; and a signaloutput terminal that is connected to an output of the first operationalamplifier, wherein an input terminal of the impedance converter isconnected to one end of the impedance element to be detected and one endof the first impedance element, and the first impedance element and theimpedance converter are included in a negative feedback loop of thefirst operational amplifier.

As a specific example, an impedance detection circuit is structured tocomprise a voltage generator, an operational amplifier of whichnon-inverting input terminal is connected to specific potential, animpedance converter, a resistance connected between the voltagegenerator and an inverting input terminal of the operational amplifier,a resistance connected between the inverting input terminal of theoperational amplifier and an output terminal of the impedance converter,and a first capacitive impedance element connected between an outputterminal of the operational amplifier and an input terminal of theimpedance converter. Impedance to be detected is connected between theinput terminal of the impedance converter and the specific potential.The specific potential in the example here indicates either certainstandard potential, specific DC potential, ground potential or afloating status, whichever suitable is selected according to a style ofan embodiment.

According to the above structure, a certain voltage is applied to theimpedance to be detected, most of electric current that flows throughthe impedance to be detected is further sent to the first impedanceelement, and then a signal corresponding to the impedance of theimpedance to be detected is output from a signal output terminal.

In order to reduce noise mixed in the signal line that connects theimpedance detection circuit and the impedance to be detected, and alsoreduce the stray capacitance between the signal line and the specificpotential, it is preferable that the signal line for the impedance to bedetected and the impedance detection circuit is as short as possible.

Also, as a type of the voltage generator, either AC or DC can beselected. Regarding characteristics of these types, the DC voltagegenerator can measure both absolute impedance and an amount changed inthe impedance, whereas the AC voltage generator only detects the amountchanged in the impedance. In the case of the DC voltage generator, sizeof its whole detection circuit becomes a little bigger since it requiresan oscillator circuit or the like. But, in the case of the AC voltagegenerator, it can remain compact because it does not require that.Therefore, according to a purpose or a use, the most suitable one can beselected for the voltage generator of the present invention. Inaddition, a second impedance element can be located between the saidfirst operational amplifier and the said voltage generator. Also, aresistance may be connected in parallel with the first impedanceelement.

Here, because the output signal from the said signal output terminalincludes a signal corresponding to voltage generated by the voltagegenerator, a cancellation unit for canceling it can further be added tothe said impedance detection circuit. An adding unit, a subtracting unitor the like can be given as an example of this cancellation unit.Especially, for a case that the impedance to be detected is capacitive,using a capacitor for the first impedance can realize a circuit thatexcels at a frequency characteristic.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing a conventional electrostaticcapacitance detection circuit.

FIG. 2 is a circuit diagram of an electrostatic capacitance detectioncircuit as an impedance detection circuit according to a firstembodiment of the present invention.

FIG. 3 A through E are diagrams showing examples of an impedanceconverter usable in the present invention.

FIG. 4 is a circuit diagram of an electrostatic capacitance detectioncircuit according to a second embodiment of the present invention.

FIG. 5 shows an example that a cancellation unit of a signalcorresponding to voltage generated by a voltage generator through anadding method shown in FIG. 4 is structured by another unit (an addingcircuit).

FIG. 6 shows an example that the cancellation unit of the signalcorresponding to the voltage generated by the voltage generator throughthe adding method shown in FIG. 4 is structured by another unit (asubtracting circuit).

FIG. 7 is a circuit diagram of the electrostatic capacitance detectioncircuit according to other embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The following provides a detailed explanation of embodiments of thepresent invention with reference to diagrams.

FIRST EMBODIMENT

FIG. 2 is a circuit diagram of an impedance detection circuit accordingto a first embodiment of the present invention. In this diagram, anelectrostatic capacitance detection circuit 10 as this impedancedetection circuit is connected to a capacitor to be detected 17 asimpedance to be detected that is a subject for detection (i.e. acapacitance type sensor that detects various types of physicalquantities using a fluctuation in the electrostatic capacitance Cs suchas a capacitor microphone in this example).

This electrostatic capacitance detection circuit 10 comprises an ACvoltage generator 11 that generates AC voltage, a resistance (R1) 12, aresistance (R2) 13, an operational amplifier 14, an impedance element 15(a capacitor with capacitance Cf in this example) and an impedanceconverter 16, and outputs a detection signal (voltage V out)corresponding to electrostatic capacitance of the capacitor 17 from asignal output terminal 20.

One end of the AC voltage generator 11 is connected to specificelectrical potential (a ground in this example), and other end (anoutput terminal) of that generates specific AC voltage (voltage V in,angular frequency ω). The resistance (R1) 12 is connected between theoutput terminal of the AC voltage generator 11 and an inverting inputterminal of the operational amplifier 14.

The operational amplifier 14 is a voltage amplifier with a high level ofinput impedance and an open loop gain, a non-inverting input terminalhere is connected to specific potential (the ground in this example),and the non-inverting input terminal and the inverting input terminalare in an imaginary short status. In a negative feedback loop of thisoperational amplifier 14, which is from an output terminal to theinverting input terminal of the operational amplifier 14, the capacitor15, the impedance converter 16 and the resistance (R2) 13 are connectedin series in this order.

The impedance converter 16 is a voltage amplifier of which inputimpedance is extremely high, output impedance is extremely low, andvoltage gain is A times. An input terminal 21 of this impedanceconverter 16 is connected to one end of the capacitor 17 via a signalline or an electric conductor such as a wiring patter on a printedcircuit board, and other end of the capacitor 17 is connected tospecific potential (the ground in this example). An output terminal ofthe operational amplifier 14 is connected to an output signal of thiselectrostatic capacitance detection circuit 10, i.e. the signal outputterminal 20 for outputting a detection signal corresponding to thecapacitance of the capacitor 17. In this patent document, a variable Aindicated for the A times or the like shows any real number other thanzero.

As for the connection between the capacitor 17 and the electrostaticcapacitance detection circuit 10, it is preferable that an unshieldedelectric conductor, which is as short as possible, (such as a cable, acopper foil wiring pattern, a connection terminal) is used, so that itis possible to prevent any unnecessary stray capacitance from beingadded as a detection error, or a disturbance noise from being mixed.Moreover, to enhance a shield against the disturbance noise, it ispreferable that a whole part of the capacitor 17 and the electrostaticcapacitance detection circuit 10 is covered with a grounded shieldmaterial or put in a shield box if possible.

Actions of the electrostatic capacitance detection circuit 10 structuredabove are as follows.

Regarding an inverting amplification circuit comprising the resistance(R1) 12, the resistance (R2) 13 and the operational amplifier 14 and thelike, both of the input terminals of the operational amplifier 14 are inthe imaginary short status and in the same potential (e.g. 0 V), theirimpedance is extremely high, and no electric current flows through, sothat the electric current passed through the resistance (R1) 12 becomesVin/R1. Because all of the electric current is passed through theresistance (R2) 13, the following expression becomes effective when theoutput voltage of the impedance converter 16 is V2.Vin/R 1 =−V 2 /R 2

When summarizing this, the output voltage V2 of the impedance converter16 can be expressed by the following expression.V 2=−(R 2 /R 1)−Vin   (Expression 1)

Also, because a voltage gain of the impedance converter 16 is A, theinput voltage V1 is expressed as follows from a relationship between theinput voltage V1 (voltage of the input terminal 21) and the outputvoltage V2 (voltage of the output terminal 22).V 1=(1/A)·V 2   (Expression 2)

When the electric current that flows from the capacitor 15 towards thecapacitor 17 is i, all of the electric current i is sent to thecapacitor 17 because the input impedance of the impedance converter 16is extremely high. Therefore, the electric current i becomes jωC·V1. Thevoltage Vout of the detection signal output from the signal outputterminal 20 is expressed as follows: $\begin{matrix}{{Vout} = {{{i \cdot ( {{1/j}\quad\omega\quad{Cf}} )} + {V1}}\quad = {( {1 + {{Cs}/{Cf}}} ) \cdot {V1}}}} & ( {{Expression}\quad 3} )\end{matrix}$

When V2 is deleted from the above expressions 1 and 2, the followingexpression is obtained.

ti V 1=−(R 2/R 1)·(Vin/A)   (Expression 4)

When this V1 is assigned to the above expression 3, the followingexpression is obtained.Vout=−(1+Cs/Cf)·(R 2/R 1)·(Vin/A)   (Expression 5)

As clarified from this expression 5, the voltage Vout of the detectionsignal output from the signal output terminal 20 of the electrostaticcapacitance detection circuit 10 becomes a value that depends on thecapacitance Cs of the capacitor 17. Therefore, the capacitance Cs can bedetermined by executing various signal processing to this voltage Vout.Also, as seen in this expression 5 where the angular frequency ω is notincluded, the voltage Vout of this detection signal does not depend on afluctuation in a frequency of the AC signal Vin from the AC voltagegenerator 11 and in a frequency of the capacitor to be detected. So, theelectrostatic capacitance detection circuit (that does not have afrequency-dependent characteristic in the circuit) capable of detectingthe capacitance of the capacitor 17 is realized without depending on thefrequency of the AC voltage applied to the capacitor 17. Therefore, forthe capacitor 17, of which capacitance value is changed at a certainfrequency (sound band), such as a capacitor microphone, it is possibleto specify a capacitance value directly from the voltage value thereofinstead of correcting the frequency for the detected signal.

Here, when the impedance converter 16 is a voltage follower, the voltagegain becomes A=1, and both of the input terminals of the voltagefollower are in an imaginary short status. So the voltage of theinverting input and the output is decided, and the voltage of thenon-inverting input of the voltage follower is determined. In this case,it can be said that the operational amplifier 14 and the voltagefollower are divided into an amplifier for obtaining enough gains and anamplifier for deciding the voltage. In this way, it becomes possible toconnect the non-inverting input of the operational amplifier 14 to thespecific potential, to improve stability of the operation, and to reduceoperational errors radically while getting enough gains, which areconsidered to be a preferable style.

Also, in the electrostatic capacitance detection circuit 10 according tothis embodiment, the operational amplifier 14, which supplies theelectric current to the capacitor 15 and the capacitor 17, has thenon-inverting input terminal connected to specific potential and fixed.Therefore, unlike the operational amplifier 95 in the conventionalcircuit shown in FIG. 1, the operational amplifier 14 can reduceoperational errors and supply stable electric current with less noisesto the capacitor 15 and the capacitor 17 without depending on thefrequency of the input AC signal or the like, and very small capacitanceof the capacitor 17 can be detected.

FIG. 3 A through E show specific circuit examples of the impedanceconverter 16 in the electrostatic capacitance detection circuit 10 shownin FIG. 2. FIG. 3 A shows a voltage follower using an operationalamplifier 100. An inverting input terminal and an output terminal of theoperational amplifier 100 are short-circuited. When a non-invertinginput terminal of this operational amplifier 100 is an input of theimpedance converter 16, and the output terminal of the operationalamplifier 100 is an output of the impedance converter 16, the impedanceconverter 16 of which input impedance is extremely high and voltage gainA is 1 can be obtained.

FIG. 3B shows a non-inverting amplifier circuit using an operationalamplifier 101. A resistance (R10) 110 is connected between an invertinginput terminal of the operational amplifier 101 and a ground, and afeedback resistance (a resistance (R11) 33) is connected between theinverting input terminal and an output terminal of the operationalamplifier 101. When a non-inverting input terminal of this operationalamplifier 101 is an input of the impedance converter 16, and the outputterminal of the operational amplifier 101 is an output of the impedanceconverter 16, the impedance converter 16 of which input impedance isextremely high and voltage gain A is (R10+R11)/R10 can be obtained.

FIG. 3C shows a circuit where a buffer of CMOS structure is added to aninput side of the operational amplifier as shown in FIG. 3A or B. Asillustrated in the diagram, N type MOSFWT34 and P type MOSFET35 areconnected between positive and negative power supplies in series via theresistance 112 and 113, and an output of the buffer is connected to aninput of the operational amplifier 100 (or 101). When the input of thisbuffer is an output of the impedance converter 16, and the outputterminal of the operational amplifier is an output of the impedanceconverter 16, the impedance converter 16 of which impedance is extremelyhigh can be obtained.

FIG. 3D shows a circuit like the buffer at the input side in FIG. 3C. Asshown in the diagram, N type MOSFET34 and P type MOSFET 35 are connectedbetween positive and negative power supplies are connected in series,and outputs are made from connection points of both MOSFET.

FIG. 3E is a circuit where a non-inverting input of an operationalamplifier 102 is an input of the impedance converter, an inverting inputterminal of the operational amplifier 102 is connected to one end of aresistance 114, and an output and the inverting input of the operationalamplifier 102 are connected via a resistance 115. As indicated in FIGS.3D and E, having these types of structure realizes the impedanceconverter 16 of which input impedance is extremely high.

SECOND EMBODIMENT

Next, the following describes an electrostatic capacitance detectioncircuit according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram of an electrostatic capacitance detectioncircuit 30 as an impedance detection circuit in the second embodiment.This electrostatic capacitance detection circuit 30 is roughly composedof a core unit 31 that is equivalent to the electrostatic capacitancedetection circuit 10 as the impedance detection circuit shown in FIG. 2,an inverting unit 32 that receives signal voltage V01 at a signal outputterminal 20 of the core unit 31 as an input and inverts the signalvoltage V01, an adding unit 33 that adds up signal voltage V03 at anoutput terminal 23 of the inverting unit 32 and signal voltage V02 at anAC output terminal 22 of the core unit 31 and outputs a detection signalof voltage V04 to an output terminal 24.

The core unit 31 has the same circuit as the electrostatic capacitancedetection circuit 10 shown in FIG. 2. Therefore, according to the aboveexpression 5, the voltage V01 of the signal output terminal 20 of thecore unit 31 is as follows:V 01=−(1+Cs/Cf)·(R 2/R 1)·(Vin/A)   (Expression 6)

According to the above expression 1, the voltage V02 of the

AC output terminal 22 of the core unit 31 is as follows:V 02=−(R 2/R 1)·(Vin/A)   (Expression 7)

The inverting unit 32 is an inverting amplification circuit comprising avariable resistance (R4) 40, a resistance (R5) 41, a variable resistance(R6) 42, a capacitor 43 and an operational amplifier 44, of whichvoltage gain is −1, and resistance values of the variable resistance(R4) 40 and the variable resistance (R6) 42 are adjusted to have a phaseof the signal V03 at the output terminal 23 identical to the one of thesignal V02 at the AC output terminal 22 of the core unit 31. Therefore,the following relation is ideally established between the input voltageV01 and the output voltage V03 of this inverting unit 32.V 03=−V 01   (Expression 8)

The adding unit 33 is an adding device of which three resistances (R7)45, (R8) 46 and (R9) 47 having the same resistance value are connectedto an operational amplifier 48. So, the following relation isestablished among two input signals of the voltage V02 and the voltageV03 and the output voltage V04.V 04=−(V 02+V 03)   (Expression 9)

After the above expression 8 is assigned to this expression 9 and V03 isdeleted, the above expressions 6 and 7 are assigned to it. Then, thefollowing expression becomes effective. $\begin{matrix}\begin{matrix}{{V04} = {{V01} - {V02}}} \\{= {{- ( {{Cs}/{Cf}} )} \cdot ( {{R2}/{R1}} ) \cdot ( {{Vin}/A} )}}\end{matrix} & ( {{Expression}\quad 10} )\end{matrix}$

Accordingly, the voltage V04 of the detection signal output from theoutput terminal 24 of this electrostatic capacitance detection circuit30 is in proportion to the capacitance value Cs. Therefore, unknowncapacitance value Cs and a fluctuation in the capacitance can be easilyspecified by executing various signal processing based on this voltageV04.

As clarified from comparison between this expression 10 and theexpression 5 that indicates the voltage Vout of the detection signalaccording to the first embodiment, unlike the first embodiment 30, thedetection signal obtained by the electrostatic capacitance detectioncircuit 30 according to the second embodiment contains only a componentbeing in proportion to the capacitance of the capacitor 17, and does notcontain any unnecessary offset (i.e. the voltage that does not depend onthe capacitor 17). Therefore, signal processing according to the secondembodiment, which specifies capacitance or a fluctuation in thecapacitance of the capacitor 17 from the detection signal, can besimple.

Although a case of V03=−V01 is used in this example, the presentinvention is not limited to this. According to a type of the capacitivesensor, the output voltage V04 can be set as follows with a case ofV03=k·V01 (k is an amplification ratio of an inverting amplificationunit).V 04={k*(Cs/Cf)+(k+1)}·(R 2/R 1)·Vin

Although the impedance detection circuit according to the presentinvention has been described based on the two embodiments and thepractical examples applied to a product, the present invention is notlimited to these embodiments and practical examples.

For instance, as shown in FIG. 5, FIG. 6 or the like, other adding unitand subtracting unit may be used for the cancellation unit of the signalcorresponding to the voltage generated by the voltage generator throughthe adding method shown in FIG. 4.

In FIG. 5, a signal output 20 (Vout) of an impedance detection circuit50, which is functionally identical to the electrostatic capacitancedetection circuit 10, is connected to one input of the adding circuitvia the resistance 46, and Vin of the impedance detection circuit 50 isconnected to other input of the adding circuit via the resistance 45.Since this signal output 20 is inverted against Vin, the signalcorresponding to the voltage generated by the voltage generator can becanceled by adding it. Compared with this, in FIG. 6, the output fromthe AC signal output terminal of the impedance detection circuit 50 andthe output from the signal output terminal Vout are used as they are.Because these two signals are inverted against Vin, as shown in thisdiagram, the subtracting circuit becomes necessary when these signalsare used as they are. The respective inputs of the adding unit and thesubtracting unit are compatibly replaceable.

Moreover, for example, in the electrostatic capacitance detectioncircuits 10 and 30, the capacitor 15 is connected between theoperational amplifier 14 and the impedance converter 16 to detect theelectric current that flows to the capacitor 17. But, instead of it, animpedance element such as a resistance or an inductance may beconnected. For example, when a resistance of a resistance value R3 isconnected in stead of the capacitor 15, the voltage Vout of thedetection signal that is output from the output terminal 20 of theelectrostatic capacitance detection circuit 10 becomes as follows instead of the above expression 5. $\begin{matrix}\begin{matrix}{{Vout} = {V01}} \\{= \begin{matrix}\{ {{( {1 + {{{R3} \cdot \Delta}\quad{{Cs} \cdot \omega}\quad{c \cdot {\cos( {\omega\quad{c \cdot t}} )}}}} ){\sin( {\omega\quad{{in} \cdot t}} )}} +}  \\{ {{{R3}( {{Cd} + {\Delta\quad{{Cs} \cdot {\sin( {\omega\quad{c \cdot t}} )}}}} )}\omega\quad{{in} \cdot \cos}\quad( {{\omega in} \cdot t} )} \} \cdot ( {{Vin}/A} )}\end{matrix}}\end{matrix} & ( {{Expression}\quad 11} )\end{matrix}$

-   -   ΔCs: for a change in the capacitance of the capacitor to be        detected    -   ωc: a frequency of the capacitor to be detected    -   Cd: invariable standard capacitance of the capacitor to be        detected    -   ωin: a frequency of the input voltage

Even in this case, there is no difference in a fact that the voltage V04of the detection signal is in proportion to the capacitance value Cs.Accordingly, an unknown capacitance value Cs and the change in thecapacitance can be easily specified by executing various signalprocessing based on this voltage V04.

Also, as shown in FIG. 7, a resistance 18 may be added and connected inparallel with the capacitor 15 in the electrostatic capacitancedetection circuits 10 and 30 according to the above embodiments. In thisway, a connecting point for the capacitor 15 and the capacitor 17 isconnected to the output terminal of the first operational amplifier 14via the resistance 18, so that having a floating status through a DCform can be avoided and the potential can be fixed.

Also, those that are connected as the impedance to be connected includeunknown capacitance (in a semiconductor chip, on a board wiring, on apackage wiring, etc.), or all of transducers (devices), which detectvarious physical quantities, such as a capacitor microphone, anacceleration sensor, a seismograph, a pressure sensor, a displacementsensor, a proximity sensor, a touch sensor, an ion sensor, a humiditysensor, a raindrop sensor, a snow sensor, a thunder sensor, a placementsensor, a bad contact sensor, a configuration sensor, an endpointdetection sensor, an oscillation sensor, an ultrasonic wave sensor, anangular velocity sensor, a liquid quantity sensor, a gas sensor, aninfrared rays sensor, a radiation sensor, a water gauge, a freezesensor, a moisture meter, a vibrometer, an electrification sensor, apublicly-known capacitive type sensor like a printed circuit boardinspection device, or the like.

As has been clarified from the above explanation, by applying AC voltageto the operational amplifier via the resistance and connecting theimpedance to be detected to the negative feedback loop of theoperational amplifier, the impedance detection circuit and theelectrostatic capacitance detection circuit according to the presentinvention detect impedance of the impedance to be detected. That is, thecapacitive impedance element is connected between the output terminal ofthe operational amplifier, of which non-inverting input terminal isconnected to the specific potential, and the input terminal of theimpedance converter, and further the impedance to be detected isconnected between the input terminal of the impedance converter and thespecific potential.

In this way, most of electric current sent to the impedance to bedetected flows to the impedance element, so that an accurate signalcorresponding to the impedance of the impedance to be detected is outputto the output terminal of the operational amplifier, which makes itpossible to detect very small impedance. Especially, when each of theimpedance is capacitive, very small capacitance that equals to or isless than a fF order can be detected, and measurement that does notdepend the frequency of the change in the capacitance to be detectedbecomes possible.

Then, because the non-inverting input terminal of the operationalamplifier is connected to the specific potential, and the potential atone end of the input terminal is fixed, the operational amplifier isfunctioned steadily, the operational error is reduced, and the noisemixed in the detection signal is restrained.

Also, since the capacitive impedance element is connected between theoperational amplifier and the impedance converter, detectionsensitivity, which does not depend on a frequency of the AC voltageapplied to the operational amplifier and on a frequency of a change inthe capacitance of the impedance to be detected, is secured. Moreover,when the resistance is connected between the operational amplifier andthe impedance converter, it does not cause a problem to degrade an S/Nratio due to thermal noise from the resistance.

Here, it is possible to add the inverting amplification circuit, whichinverts a signal at the signal output terminal, and the adding circuit,which adds up the output signal of the impedance converter and theoutput signal of the inverting amplification circuit, to the saidimpedance detection circuit. By doing so, any unnecessary offsetcomponent contained in the output signal of the impedance detectioncircuit is removed, and a net signal corresponding to the impedance ofthe impedance to be detected can be amplified significantly.

As has been mentioned, the present invention reduces limitation for ausage environment, detects very small impedance accurately, and realizesan impedance detection circuit and an electrostatic capacitancedetection circuit and the like that are suitable for miniaturization,and especially sound performance of lightweight and compact audiocommunication devices such as a mobile phone is rapidly improved and itspractical value is extremely high.

Industrial Applicability

The electrostatic capacitance detection circuit according to the presentinvention may be used as a detection circuit of a capacitance typesensor, especially as a microphone device that is equipped with compactand lightweight devices such as a mobile phone.

1. An impedance detection circuit that outputs a detection signalcorresponding to impedance of an impedance element to be detected,comprising: an impedance converter of which input impedance is high andoutput impedance is low; a first capacitive impedance element; a firstoperational amplifier; a voltage generator that applies at least ACvoltage or DC voltage to the first operational amplifier; and a signaloutput terminal that is connected to an output of the first operationalamplifier, wherein an input terminal of the impedance converter isconnected to one end of the impedance element to be detected and one endof the first impedance element, and the first impedance element and theimpedance converter are included in a negative feedback loop of thefirst operational amplifier.
 2. An impedance detection circuit thatoutputs a detection signal corresponding to impedance of an impedanceelement to be detected, comprising: an impedance converter of whichinput impedance is high and output impedance is low; a first impedanceelement; a first operational amplifier; a voltage generator that appliesat least AC voltage or DC voltage to the first operational amplifier; asignal output terminal that is connected to an output of the firstoperational amplifier; and a cancellation unit that cancels all or apart of a signal, which corresponds to voltage generated by the voltagegenerator, from a signal at the signal output terminal, wherein an inputterminal of the impedance converter is connected to one end of theimpedance element to be detected and one end of the first impedanceelement, and the first impedance element and the impedance converter areincluded in a negative feedback loop of the first operational amplifier.3. The impedance detection circuit according to claim 1 or 2, whereinthe impedance element to be detected is a capasitive impedance element.4. The impedance detection circuit according to any of claims 1 through3, further comprising a resistance element connected in parallel withthe first impedance element.
 5. The impedance detection circuitaccording to any of claims 1 through 4, further comprising a secondimpedance element connected between the first operational amplifier andthe voltage generator.
 6. The impedance detection circuit according toany of claims 1 through 5, wherein the one end of the impedance elementto be detected and the input terminal of the impedance converter areconnected each other by an unshielded conductor.
 7. The impedancedetection circuit according to any of claims 1 through 6, wherein theimpedance converter is a voltage follower of which voltage gain is one.